1. Field of the Invention
The invention relates to a signal level detector for a compact disc player and, more particularly, to a level detector furnishing an output for controlling the position of the beam relative to the track.
2. Description of the Prior Art
Such a level detector may be employed in, for example, a Compact Disc Audio player. At the time of filing of the present Application such a player is commercially available under the type designation Philips CD 303. In such a player the digital audio information in the form of a spiral tracks of pits and lands is read by means of a laser beam and converted into a signal current. This signal current constitutes a sequence of sinusoidal pulses of variable width. The signal is amplified and applied to inter alia a plurality of level detectors, in which the signal level of the rectified signal is compared with a plurality of reference levels in order to supply control signals for the various control devices employed in a compact-disc player if these reference levels are not reached. The player has, for example, a tracking system in order to keep the laser beam on the track. The maximum signal level is obtained if the laser beam follows the center of the track. If the beam is offset from this center, the signal level decreases. If a level corresponding to approximately 60% of the nominal level is not reached, a level detector generates a control signal by means of which the laser beam is returned to the center of the track. In addition, the player has level detectors which, if a level corresponding to approximately 50% and 20% of the nominal level is not reached generate a control signal for respectively disabling a phase-locked loop in the speed-control system, which system becomes unreliable for low signal levels, and for detecting the absence of the signal as a result of flaws in and on the disc, so called drop-outs. For these control systems it is important that the level detectors accurately detect whether the levels are reached and in such a way that brief interruptions in the signal level between two consecutive pulses and spurious pulses cannot actuate these control systems.
In the known compact-disc player the level detectors are constructed as a discrete circuits with non-integrable capacitors. However, it may be desirable to integrate these detectors on the same substrate as the other control circuits. A level detector which, in prinicple, is suitable for this purpose is known from U.S. Pat. No. 4,338,530.
In this detector the input signal is applied to a comparator which compares the input signal level with a reference level. The output signal of this comparator is applied to a first delay circuit which ensures that if the input signal briefly decreases below the reference level of the comparator such a decrease does not produce an output signal on the output of the level detector. The level detector only produces an output signal on its output when the input signal has remained below the reference level of the comparator for a specific time interval. A second delay circuit ensures that when the input signal again exceeds the reference level of the comparator the output signal of the level detector is sustained until the input signal has remained above the reference level for a specific second time interval.
A drawback of this detector is that if the first time interval and the second time interval are equal to each other, which is desirable for the above-mentioned uses, a positive spurious pulse during a detected decrease below the reference level will be interpreted as an increase above the reference level. This leads to an undesired actuation of further circuits.